|Maps of Glacier Recession, Muir Inlet
|Types of Glaciers||These
maps, both compiled by Bill Fields, show retreat of the Muir Glacier from 1880
until the 1980s. The record has subsequently been extended by satellite measurements.
Even though the Muir has been shrinking for over 200 years, it is not a good indication that
the Earth's climate is warming. A tidewater glacier, it's rate of growth or recession is
determined by the calving of icerbergs at the terminus. A nearby tidewater glacier, the
Great Pacific, has been growing rapidly.
(Top map from the American Geographic Society Collection archived at the National Snow and Ice Data Center, University of Colorado at Boulder. Bottom map Hall et. al., 1995)
|Types of Glaciers|
Glaciers form when snow accumulates on a patch of land over tens to hundreds of years. The snow eventually becomes so thick that it collapses under its own weight and forms dense glacial ice. When enough of the ice is compacted together it succumbs to gravity and begins to flow downhill or spread out across flat lands (Williams and Hall, 1993). There are many different types of glaciers, and not all of them are good indicators of climate change. "Glaciers that do tend to be good climate indicators are small land-based, non-surge type glaciers. They respond directly to both regional temperature and precipitation [snow]," said Dorothy Hall, a hydrospheric scientist at NASAs Goddard Flight Space Center. She and a team of scientists from around the globe have used satellites over the past 25 years to measure changes in glaciers in Europe, Iceland and Alaska.
More than 90 percent of the 33 million cubic kilometers of glacier ice in the world is locked up in the gigantic Greenland and Antarctic ice sheets. Because they are so massive and exist in such frigid latitudes, large-scale changes are very difficult to track and verify, said Hall.
In addition to the ice sheets, there are two types of small glaciers that
make for bad climate gauges. "Both surge glaciers and tidewater glaciers
have their own cycles of advance and retreat. These cycles are certainly
related to climate, but we are not exactly sure how," said Hall. Even if
the climate changes in the region, these glaciers would most likely maintain
their distinctive patterns of behavior.
Tidewater glaciers terminate in the ocean, where the boundary can be marked by awesome ice cliffs 200 feet high. When huge blocks of ice break off to form icebergs (a process known as calving) the waves can be large enough to capsize ships. (Photograph by Bill Field, 1971)
|Surge glaciers can sluice down a
valley at rates of up to a few kilometers a day, said Hall.
Once they attain their final destination, they stagnate or gradually retreat for
the next ten-to-fifty years. Tidewater glaciers, on the other hand, advance for
roughly a thousand years before reaching their destination, explained Hall.
When they encounter the sea, they calve and drop icebergs into the water as they
continue to make their way out into the ocean. They then pull back and retreat
over the course of one or two hundred years.
The Muir tidewater glacier in Alaskas Glacier Bay has been observed for nearly 200 years. Explorers in the mid-1700s recorded observations of Muir at its peak. They noticed that about two hundred years ago it began to recede. Recent measurements show that the glacier has withdrawn more than 90 kilometers (Hall et. al., 1994).
This map, compiled by the glaciologist Bill Field, shows the changes in the glaciers of Muir Inlet, Alaska, from 1880 to 1941. Large map. (Map from the American Geographic Society Collection archived at NSIDC)
|Using satellites to measure glaciers|
|The trick to measuring the extent of a glacier using a satellite image
lies in distinguishing the glaciers edge from the surrounding land.
Unfortunately, this task isnt as simple as drawing a line on the image
between what looks like a glacier and what looks like land. Both glaciers and
the surrounding ground often have the same dark gray coloring and can be easily
confused by sight.
||This aerial photograph
shows one of the problems inherent in studying glaciers with remote sensing. The dark
bands on the surface of the glacier, caused by ground-up rocks, make it difficult to distinguish
the glacier's edges from the surrounding terrain.
Barnard Glacier, AK · Austin Post Photo · July 29, 1957
(Photograph from the American Geographic Society Collection archived at the National Snow and Ice Data Center, University of Colorado at Boulder.)
|To separate glacier from surrounding land more
accurately, scientists must look at specific types of light being reflected. As
can be seen through a prism, sunlight contains many different individual colors
(wavelengths). When sunlight strikes objects, certain colors of the spectrum
are absorbed and others are reflected. The reflected wavelengths give an object
Like most white objects, the glacier reflects nearly all the
colors of the visible spectrum, including the yellow-green sunlight. Yet,
a glacier absorbs near-infrared wavelengths of solar energy (light to the right of red
on the color spectrum). The research teams using the Landsat 5 and Landsat 7
satellites differentiate snow from other solid materials by looking at the
difference between the infrared and yellow-green wavelengths that are reflected.
Whenever the difference is large, the area in question is likely to be snow or
glacier ice (Hall et al. 1998). By tracking the edges of a glacier from year to
year, scientists are often able to see if it is receding or advancing.
Hall explained, while satellite data are easier to collect than ground measurements, scientists still have to record images for many years before they can be certain a glacier is changing, and it is critical to abtain ground measurements to corroborate results deduced from analysis of satellite data. The most obvious reason for the scientists uncertainty has to do with the "resolution" of the satellite images. The Landsat 5 and Landsat 7 satellites have a resolution of 30 meters, which means that each pixel (or picture element) on the image represents a 30-by-30-meter patch of land. "Though the glacier will respond immediately to the changes in climate, it may take five to twenty years before we can see the changes in the glacier from satellites," said Hall.
|This pair of images from the Landsat series of satellites, taken more than a decade apart, shows the continuing retreat of the Muir glacier. The front face of the glacier is marked by the arrow in each image, and moved more than 7km between 1973 and 1986. (Image courtesy Dorothy Hall, NASA Goddard Space Flight Center)|